1. Field of the Invention
Exemplary embodiments of the present invention relate to an alternating current (AC) light-emitting diode (LED) dimmer and a dimming method thereby and, more particularly, to an AC LED dimmer which performs a dimming function of an AC LED by bidirectionally switching AC voltage at a high speed under control of a pulse width modulation signal, and a dimming method thereby.
2. Discussion of the Background
In general, a lamp may have a dimming function that allows a user to control brightness of the lamp, but such a function has been restrictively used in practice. As energy saving has become an important concern in association with an increase in electrical energy consumption, the dimming function of the lamp becomes an essential function, rather than an optional function, to save energy. Further, a light-emitting diode (LED) has attracted attention as an environmentally friendly lighting source to save electric energy.
A conventional representative dimmer dims an AC LED by adjusting the root-mean-square (RMS) value (Vrms) of AC voltage by controlling the AC phase of the AC voltage using a semiconductor device such as a triode for alternating current (Triac).
A Triac is an electronic component approximately equivalent to two silicon-controlled rectifiers (SCRs/thyristors) connected to each other in inverse parallel (parallel but with the polarity reversed) and with their gates connected together. The Triac can be triggered by either a positive or a negative voltage applied to its gate electrode, and, once triggered, it continues to conduct until the current through the Triac drops below a certain threshold value. Triacs are well known in the art and a detailed description thereof will be omitted herein.
Such a phase control scheme adjusts the RMS value of output voltage by driving the Triac after a predetermined delay from when an input voltage is 0V (at the moment when the input voltage starts to rise or decrease). However, the phase control scheme and the traditional dimming method using the Triac are limited in terms of operating range due to a controller configured to drive the Triac and inherent characteristics of the Triac.
The traditional dimmer and dimming method will be described with reference to the accompanying drawings.
FIG. 1 is a block diagram of a traditional dimmer using a Triac. The dimmer 10 includes a Triac 14 and a resistor/capacitor (R/C) phase controller 16. The Triac 14 supplies or blocks AC voltage from an AC voltage source 12 to a lamp, i.e. an AC LED 18, and the R/C phase controller 16 controls the Triac 14. Hence, the Triac 14 is turned on by a gate turn-on signal g from the R/C phase controller 16 to allow the AC voltage to be supplied to the AC LED 18.
The dimmer 10 generates a phase control signal, i.e., gate turn-on signal IG, using a resistor R and a capacitor C, when the AC input voltage is 0V, to drive the Triac 14. The phase control signal is an AC voltage signal delayed by a time constant determined by the resistor and the capacitor.
Considering the operating characteristics of a typical Triac, the dimming range of the dimmer 10 is limited depending on the drive voltage of the Triac.
FIG. 2 is a waveform graph of AC input voltage v1 and AC input current i1 in the traditional dimmer in FIG. 1. Referring to FIG. 2, the phase control scheme using the Triac leads to a non-sinusoidal waveform of the current i1.
When the AC input voltage is 0V, a phase control signal, i.e., gate turn-on signal IG in FIG. 1, which is generated using the resistor R and the capacitor C, causes the Triac 14 (see FIG. 1) to abruptly conduct current due to the operation characteristics of the Triac, thereby resulting in the non-sinusoidal waveform of the current i1, as shown in FIG. 2. Further, a time point when the current i1 starts to flow in the current waveform depends on the resistor and the capacitor of the R/C phase controller 16. In determining such a phase delay, an operating margin of the resistor and the capacitor is required. An insufficient operating margin may cause the gate turn-on signal IG (see FIG. 1) to instantaneously flow, thereby causing the AC LED to flicker.
As such, there is a problem in that a minimum dimming range and a maximum dimming range are very limited due to the drive voltage of the Triac and the characteristics of the resistor and capacitor of the R/C phase controller.
In addition, the Triac is abruptly switched by the gate turn-on signal in the phase control scheme using the Triac, thereby producing a number of harmonics during the switching process (especially, turn-on time denoted by reference numeral 20 in FIG. 2).
Accordingly, a new AC voltage source driver and controller are needed to achieve a broader control range and a linear dimming function.